The present invention relates to improvements in increasing the radiopacity of stents and improvements in their method of manufacture, and, more particularly, to a stent and a method of manufacture where radiopaque material is secured to strategic location(s) on the stent to improve visibility of the stent under fluoroscopy.
Generally, stents are expandable endoprosthesis devices which are adapted to be implanted into a patient""s body lumen to maintain the patency of the vessel. Stents are especially well-suited for the treatment of atherosclerotic stenosis in blood vessels. These devices are typically implanted into blood vessels by a delivery catheter which is inserted at an easily accessible location and then advanced through the patient""s vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is usually deployed either automatically by the removal of a restraint, or actively by the inflation of an expandable member, such as balloon, about which the stent is mounted on the delivery catheter.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads that are imposed by the vessel walls. In addition to having adequate radial strength, the stent should be longitudinally flexible to allow it to be maneuvered through a vascular path and to enable it to conform to a deployment site which may not be linear or may flex. The stent material must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent""s structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
Fluoroscopy is typically used to facilitate the precise placement of a stent as well as to verify the position of a stent within the patient throughout the stent""s service life. The use of radiopaque materials in the construction of the stent allows for its direct visualization. Accordingly, different patterns and contents of radiopactivity have different effects on the direct visualization. For example, when a physician views a completely radiopaque stent under fluoroscopy, he/she will likely see an unclear and amorphous shape that extends outside the dimensions of the actual stent. The opposite will also be true where the stent possesses little radiopacity. In terms of fluoroscopic visibility, the optimal stent should be visible in a clear and detailed form without shape blurring. To date, no single material has been identified that simultaneously satisfies all requirements inherent in an optimal stent application. Those materials that do satisfy the mechanical requirements are either insufficiently or excessively radiopaque and/or are not adequately proven to be biocompatible in a vascular setting. Thus, simply constructing a stent which exhibits optimal radiopacity wholly out of a single material is not currently a viable option. A number of different approaches have, however, been employed wherein different materials are combined in an effort to render a mechanically sound and biocompatible stent to be visualized by a fluoroscope.
One procedure frequently used for accomplishing fluoroscopic visibility is through physical attachment of radiopaque markers to the stent. Conventional radiopaque markers, however, have a number of limitations. When attached to a stent, such markers may project from the surface of the stent, thereby exhibiting a departure from the ideal profile of the stent. Depending on their specific location, the marker may either project inwardly tending to disrupt blood flow or outwardly tending to traumatize the walls of the blood vessel. Additionally, when the metal used for the stent structure comes in contact with the metal used for the radiopaque marker, galvanic corrosion may occur. This corrosion may lead to separation of the metals and thereafter contamination of the blood stream with radiopaque material. Additionally, the radiopaque material may come into direct contact with living tissue which may be problematic, particularly if the material is not biocompatible.
Stents can also be marked by plating selected portions thereof with radiopaque material. A number of disadvantages with this approach are apparent. Because the radiopaque material comes into direct contact with living tissue, there can be a sizeable amount of tissue exposure. Additionally, when the stent is expanded and certain portions undergo substantial deformation, there is a risk that cracks will form in the plating which can separate from the underlying substrate. This side effect has the potential for creating jagged edges on the stent which may inflict trauma on the vessel wall or cause turbulence in the blood flow thereby inducing thrombogenesis. Moreover, once the underlying structural material becomes exposed, interfaces between the two disparate metals become subject to the same type of galvanic corrosion as mentioned above. Further, should the plating pattern cover less than all of the stent""s surfaces, the margins between the plated and unplated regions are all subject to galvanic corrosion.
As a further alternative, a stent structure can be formed from a sandwich of structural and radiopaque materials. Tubes of suitable materials can be cold-drawn and heat treated to create a multi-layered tubing which can be cut into a stent. Struts and spines are formed in the multi layered tubing by cutting an appropriate pattern of voids into the tubing using well known techniques in the art. While this approach does provide a stent that has enhanced radiopacity and techniques fulfills necessary mechanical requirements, the thin cross section of the radiopaque material is usually exposed along the edges of all cut lines. The biocompatiblity of the radiopaque material remains an issue and more significantly, a sizeable area is created which is subject to galvanic corrosion. Any cuts in the sandwich structure cause two disparate metal interfaces, i.e. the juncture between the outer structural layer and the central radiopaque layer, as well the juncture between the central radiopaque layer and the inner structural layer, to become exposed to blood and tissue along the entire lengths of these cuts.
A stent configuration that overcomes the shortcomings inherent in previously known devices is therefore required. More specifically, a stent is needed that provides radiopaque properties enabling clear visibility under fluoroscopy and mechanical properties consistent for reliable and safe use.
A method of manufacturing the above mentioned stent configuration is also necessary. More specifically, a method is needed that combines the prerequisites of biocompatible materials and fluoroscopy into an advantageous method of manufacture.
The present invention provides a stent and method for manufacture which overcomes some of the shortcomings of previously known stents and methods of manufacturing stents. Most importantly, the stent has high resolution when viewed under fluoroscopy due to strategic placement of radiopaque material along the stent. The stent also fulfills the requirements of having sufficient flexibility, structural integrity and biocompatiblity, and being safe for deployment into a patient""s vasculature.
Unique to the stent described herein is an advantageously selected pattern of radiopaque material formed on the stent. Compared to conventional stents, which are frequently obscured under fluoroscopy, the pattern of radiopaque material in the stent described herein leads to improved visibility under fluoroscopy. Unique to the process is a method of forming selected patterns of securely mounted radiopaque material on a stent substrate. Compared to some conventional processes of forming stents where radiopaque material is simply layered onto the stent structure, the process described herein utilizes a process of placing select patterns of grooves into the stent substrate. This grooving process allows precise placement and strong retention of radiopaque material into strategic locations of the stent.
The material employed for said underlying structure is selected for its structural and mechanical properties and may be the same general material used to make conventional stents. One preferred material is 316L stainless steel, although other materials such as nickel-titanium, cobalt based alloys, Nitinol and other types of stainless steel can be used. Such materials, when used in the 0.002xe2x80x3 to 0.003xe2x80x3 thickness, as is typical for stent applications, are often difficult to visualize fluoroscopically.
The grooving process is the first operation to be performed on a piece of tube stock. In this process, apattern of groove(s), preferably either rings or lines, is cut in the tube stock. The grooves should be strategically placed at targeted locations along the length of the tubing to obtain the desired radiopacity. The stent may employ one or a multiple number of grooves depending on radiopacity requirements. In forming the groove(s), an instrument, such as a conventional Swiss screw machine can be used. Alternative machines can also be used to perform the same grooving operation.
After the grooving process is performed on the tube stock, radiopaque material is inserted into the groove(s) by either press-fitting, diffusion bonding or laser bonding. One preferred radiopaque material is gold, although other radiopaque materials such as platinum, tantalum, iridium, or their alloys can also be used. When press-fit, the shapes of the strip(s) must be in close conformance with the shape of the groove(s) while being slightly larger than the size of the groove(s). The radiopaque strip(s) are combined with the groove(s) in the tube such that the difference in sizing causes the two metals to lock together in an interference fit. The interference fit insures a strong and long lasting bond between the two materials. When the radiopaque strip(s) are diffusion bonded, an entirely different process of attachment can be employed. In the diffusion bonding procedure, a vacuum is drawn and the entire assembly (tubing with radiopaque material inserted into the groove(s)) is heated to near the particular diffusion bonding temperature with the bonding surfaces still exposed to the vacuum environment. Thereafter, the bonding surfaces are brought into contact with very moderate pressure and maintained at a temperature and pressure sufficient for diffusion bonding. The assembly is then cooled, resulting in a substantially unitary diffusion bonded structure.
While not mandatory for this process, stainless steel can be applied over the radiopaque sections of the tube to promote biocompatibility and structural integrity. Additionally, the stainless steel coating can act to protect the radiopaque and structural materials from galvanically corroding. The stainless steel, preferably 316L can be applied by a sputtering procedure, a method of depositing a metallic film through the use of electric discharge. Sputter coating machines are commercially available and capable of applying an extremely even coating of material to a workpiece. The tubing may be rotated in front of a nozzle, the nozzle may be rotated about the tubing or a nozzle that completely surrounds the tubing may be employed to apply the sputter coating. While the preferred material for the sputtering is 316L stainless steel, other suitable material can be used also. Additionally, if a higher degree of structural rigidity is sought, the material can be sputtered across the entire length of the tube to a sufficient thickness such that the structural integrity of the stent is significantly increased.
After the tube has been processed as described above, a procedure for cutting the tube can be initiated. In this procedure, for example, the tubing is first placed in a rotatable fixture inside a cutting machine, where it is positioned relative to a laser. According to machine encoded instructions, the tubing is rotated and moved longitudinally relative to the laser. The laser selectively removes the material from the tubing by ablation and a pattern is cut into the tube. The laser cut provides a desired pattern of voids defining struts and spines which allows the stent to expand in an even manner, in accordance with well known and well established procedures. Thereafter, the stents are subjected to the standard industry practices of electro-polishing and possibly annealing. Another biocompatible outer layer could also be applied to the stent.